For production of fresh water from seawater or for production of clean water from river or lake water, reverse osmosis membrane apparatuses having a reverse osmosis membrane module, for example, are used. In a reverse osmosis membrane apparatus, a to-be-treated water (clarified water) produced by subjecting e.g. sea, river or lake water as raw water to pretreatment such as sterilization treatment of adding a fungicide into intake sea, river or lake water or a treatment of removing impurities by using e.g. a sand filter, is used. The to-be-treated water is pressurized to have a pressure of about 6.0 MPa by a high pressure pump, for example, and is supplied to a reverse osmosis membrane module, and the to-be-treated water is permitted to pass through a reverse osmosis membrane by reverse osmosis effect by the reverse osmosis membrane module to obtain permeate water to be production water.
A reverse osmosis membrane module is usually composed of a plurality of reverse osmosis membrane elements. An example of the reverse osmosis membrane elements is a spiral-type reverse osmosis membrane element having a structure where a sac-like reverse osmosis membrane having a flow path material therein is spirally wound via a mesh spacer around a center pipe in which permeate water is collected, and a brine seas is provided at an end of the outer peripheral surface, as disclosed in Patent Document 1. Another example of the reverse osmosis membrane elements is a flat membrane-type reverse osmosis membrane element having stacked a plurality of flat sheets of reverse osmosis membrane.
A reverse osmosis membrane module usually has a high pressure vessel and a plurality of reverse osmosis membrane elements arranged in series in the high pressure vessel, and to-be-treated (clarified water) is separated with reverse osmosis membrane elements sequentially from one on the most upstream side into permeate water and concentrated water containing saline matters and impurities to obtain permeate water by each of the reverse osmosis membrane elements. The concentrated water separated from the permeate water by each of the reverse osmosis membrane element is further separated into permeate water and concentrated water by a later stage element of the reverse osmosis membrane elements. Therefore, the salinity and concentration of impurities of concentrated water are higher at relatively downstream side. Patent Document 2 discloses a construction where a plurality of high pressure vessels each having a spiral-type reverse osmosis membrane element therein, is connected to a to-be-treated water introducing passage and arranged in parallel.
FIG. 6 shows an example of a construction of a reverse osmosis membrane module having a high pressure vessel and spiral-type reverse osmosis membrane elements arranged in series therein, which is disclosed in Patent Document 1. In FIG. 6, the reverse osmosis membrane apparatus 100 comprises a high pressure vessel 102 and four to eight reverse osmosis membrane elements 104 arranged in series in the high pressure vessel 102. To-be-treated water tw is supplied at a high pressure to an inlet opening 102a of the high pressure vessel 102 from a to-be-treated water supply passage 114. If it is assumed that among the plurality of reverse osmosis membrane elements arranged in series, a reverse osmosis membrane on an upstream side of the flow of the to-be-treated water tw is an earlier stage element and a reverse osmosis membrane on a downstream side of the flow of the to-be-treated water tw is a later stage element, the to-be-treated water tw is flown into an inlet end of the earliest-stage element of the reverse osmosis membrane elements 104 and separated into permeate water pw and concentrated water cs by the reverse osmosis membrane in the reverse osmosis membrane element 104a. 
The permeate water pw is flown into the center pipe 106, and the concentrated water cw is flown out from the outlet of the earliest-stage reverse osmosis membrane element. The inlet of the center pipe 106 of the earliest-stage reverse osmosis membrane element 104a is obstructed with an end cap 108. The center pipe 106 of each of the reverse osmosis membrane elements 104 is connected with a connector 110. Accordingly, the permeate water pw from each of the reverse osmosis membrane elements joins together and is discharged from an outlet opening 102b of the high pressure vessel 102 to a permeate water outflow passage 116.
As the interior of the high pressure vessel 102 is partitioned with brine seals 112 provided on outer peripheral surface of each of the reverse osmosis membrane elements 104, the concentrated water cw flown out of each of the reverse osmosis membrane elements 104 is flown into the later stage element of the reverse osmosis membrane elements as to-be-treated water without going past the reverse osmosis membrane element of the later stage. The concentrated water cw is thereby permeated sequentially with the reverse osmosis membrane elements. The concentrated water cw discharged from the reverse osmosis membrane element 104 in the last stage is discharged from an outlet opening 102c formed at the outlet end of the high pressure vessel 102 to a concentrated water discharging passage 118.
A differential pressure meter 120 for detecting the differential pressure between the pressure of the to-be-treated water tw flowing in the to-be-treated water introducing passage 114 and the pressure of the concentrated water cw flowing in the concentrated water discharging passage 118 is provided.
The method of washing the reverse osmosis membrane element 100 was such that when the detected value by the differential pressure meter 120 exceeds the threshold deposition amount of fouling matters such as hardly-soluble ingredients, high molecular solute, colloids, or tiny solid materials deposited in the reverse osmosis membranes is defined as exceeding the threshold limit, and then the operation of the reverse osmosis membrane apparatus 100 was stopped, and washing was carried out. When the differential pressure between the to-be-treated water tw flowing in the to-be-treated water introducing passage 114 and the permeate water pw flowing in the permeate water outflow passage 116 exceeds the threshold, the same washing method as described above may be applied. Accordingly, the difference in pressure between the to-be-treated water tw flowing in the to-be-treated water introducing passage 114 and the permeate water pw flowing in the permeate water outflow passage 116 may be detected.